The present invention relates to a semiconductor photodetecting device and an illuminance sensor used in visibility illuminance measurement or the like.
There has recently been a growing demand for an illuminance sensor having a spectral sensitivity characteristic close to a human visibility characteristic. Such an illuminance sensor can be used for controlling that when the amount of light applied to the illuminance sensor is large as in daylight and in the case of a bright room with the illuminance sensor being mounted to a cellular phone, for example, the backlight of a keypad is turned off and the backlight of a liquid crystal panel is raised to the maximum brightness, whereas when the amount of light applied to the illuminance sensor is small as in the open air at night, the backlight of the keypad is turned on and the backlight of the liquid crystal panel is diminished or reduced. Controlling the turning on of the backlight and its brightness according to the ambient brightness using the illuminance sensor in this way makes it possible to reduce the drain on a battery and make continuous talk time and standby time longer.
The peak value of a spectral sensitivity or response characteristic of such an illuminance sensor is appreciably approaching 555 nm corresponding to the peak value of human visibility such as on the order of about 550 nm to 600 nm. There is however a case in which a defective condition occurs in that output currents differ even under the same illuminance where a source of light is a fluorescent light and sunlight, for example. This is ascribable to the fact that a difference occurs between emission spectra depending on the source of light, and the illuminance sensor has sensitivity even with respect to infrared light and ultraviolet light imperceptible to the human eye. That is, in order to allow the sensitivity of the illuminance sensor to coincide with the human visibility characteristic, there is a need to match spectral sensitivity characteristics on the long-wavelength and short-wavelength sides as viewed from the peak value with the human visibility as well as the matching of peak wavelengths of spectral sensitivity characteristics.
In order to solve such a problem, a patent document 1 (Japanese Unexamined Patent Publication No. 2006-245264) (FIG. 5) has shown an example of a photosensor or photodetecting device which eliminates an influence on the output current based on the infrared light and has a spectral sensitivity characteristic corresponding to visibility. Described specifically, the patent document 1 has described that two p-type regions 34 and 35 are respectively formed in an n-type semiconductor layer 2 at depths of 1.5 μm and 3 μm thereby to form two diodes D4 and D5, the output of the diode D5 is set to 1.05 times by an arithmetic circuit and the resultant value is subtracted from the output of the diode D4 thereby to enable the sensitivity of an infrared region to be approximately zero, thus making it possible to realize the spectral sensitivity characteristic corresponding to the visibility.
It has been described in the patent document 1 that the outputs of the two diodes different in spectral sensitivity are subjected to arithmetic processing in the photodetecting device described in the patent document 1 thereby to set the sensitivity of the infrared region to approximately zero. A graph indicating the dependence of each output current on the wavelength is shown in FIG. 6 of the same patent document. According to the graph, the sensitivity still exists on the short-wavelength side although an improvement effect appears in the infrared region. It is hard to say that this is one corresponding to the human visibility characteristic. There is thus room for its improvement.